1. Field of the Invention
The present invention relates to a cast strip withdrawing apparatus for a continuous casting facility. More specifically, the present invention relates to a cast strip withdrawing apparatus for a continuous casting facility that does not allow an excessive pressing force to act on a cast strip or dummy bar during normal operation, and does not allow a cast strip or dummy bar to fall when there is hydraulic trouble.
2. Description of the Related Art
Conventionally, in continuous casting facilities that continuously cast molten steel supplied from a ladle, a plurality of roll segments is arranged below a mold along a withdrawing direction (casting direction) of cast strips, and a cast strip guide device is configured by this plurality of roll segments. Every roll segment is made to include a support roll that supports the cast strip thus cast, while between these roll segments, a plurality of cast strip withdrawing apparatuses equipped with a drive roll for withdrawing the cast strip along the withdrawing direction is installed.
Furthermore, it is configured so that the cast strip thus cast is withdrawn by the cast strip withdrawing apparatuses while being supported by the support rolls. However, at the initial stage of operation of a continuous casting facility, ahead of the cast strip, a dummy bar is withdrawn by the cast strip withdrawing apparatus while being supported by the support rolls.
Therefore, the cast strip withdrawing apparatus of the continuous casting facility related to the conventional example will first be explained while referring to the following appended FIGS. 9 to 12 as well. FIGS. 9 and 10 are a plan view and front view showing a cast strip withdrawing apparatus of the continuous casting facility according to conventional example 1, respectively. FIG. 11 is a hydraulic circuit illustrative drawing that includes an emergency hydraulic valve of a cast strip anti-drop device in a continuous casting machine according to conventional example 2. FIG. 12 is a schematic diagram showing an example of a cast strip and dummy bar anti-drop hydraulic circuit for a continuous casting machine according to conventional example 3.
Therefore, the cast strip withdrawing apparatus of the continuous casting facility according to conventional example 1 will first be explained hereinafter while referring to the appended FIGS. 9 and 10. Normally, plural cast strip withdrawing apparatuses 33 are provided at predetermined intervals in the vertical direction in a vertical continuous casting facility. In each cast strip withdrawing apparatus 33, two pairs of fixed-side rolls 37 are mounted to each frame 36, pressure-side rolls 38 are mounted to be able to advance and retract with respect to the fixed-side rolls 37, and a cast strip 32 is held between both the rolls 37 and 38 by pressing the pressure-side rolls 38 toward the fixed-side rolls 37 by way of a hydraulic cylinder 39. Then, both the rolls 37 and 38 are rotationally driven by a drive motor 48 via worm reduction devices 41 and 42, universal spindles 46 and 46, and a reduction mechanism 47, whereby the cast strip 32 held between both the rolls 37 and 38 is withdrawn (refer to Japanese Examined Patent Application Publication No. H2-32062).
Here, in order to facilitate understanding, among the cast strip withdrawing apparatuses 33, a cast strip withdrawing apparatus 33 disposed at the upstream most side is defined as an upstream-most side withdrawing apparatus, and a cast strip withdrawing apparatus 33 after this withdrawing apparatus is defined as a downstream-side withdrawing apparatus, and although their drawings are omitted, the names and symbols will be explained distinctly.
Prior to the start of casting, the dummy bar is first retained by the upstream-most side withdrawing apparatus; whereas, when casting is started, until the dummy bar is handed over from the upstream-most side withdrawing apparatus to the downstream-side withdrawing apparatus, the dummy bar and the cast strip 32 following this are retained by the upstream-most side withdrawing apparatus. During this time, there is no means for supporting the dummy bar and cast strip 32 other than the upstream-most side withdrawing apparatus.
There is similarly no means for supporting the dummy bar and cast strip 32 other than the downstream side withdrawing apparatus, after the dummy bar has been handed over to the downstream side withdrawing apparatus. A method increasing the retaining force by clamping the cast strip 32 by way of the upstream-most side withdrawing apparatus in addition to the downstream side withdrawing apparatus clamping the dummy bar can be considered; however, since the thickness of the solidified shell of the cast strip 32 on the upstream side is thin, clamping the cast strip 32 with an excessive pressing force by the upstream-most side withdrawing apparatus is not preferable from the viewpoint of the quality of the cast strip 32. Therefore, it is preferable to support the dummy bar by way of the downstream side withdrawing apparatus until the dummy bar is handed over to a cast strip withdrawing apparatus on a further downstream side of the downstream side withdrawing apparatus.
As can be understood from FIGS. 9 and 10, similarly to the cast strip 32, the dummy bar, which is not illustrated therein, is retained by the friction force generated at the contact surface between the pressure-side roll 38 and the dummy bar, and the contact surface between the fixed-side roll 37 and the dummy bar. This friction force is proportional to the pressing force of the hydraulic cylinder 39. The friction force (i.e. retaining force) is obtained by multiplying the hydraulic cylinder pressing force by the coefficient of friction of the contact surfaces between both the rolls 37 and 38 and the dummy bar.
If the retaining force is insufficient, the dummy bar falls. Especially in such a vertical continuous casting facility according to conventional example 1, the falling distance is long as tens of meters, and the dummy bar vertically falls differently from fall along a slope in a curved continuous casting facility. As a result, the fall energy becomes extremely large, and the falling trouble of the cast strip 32 and dummy bar is fatal damage accompanying considerable equipment damage.
Furthermore, in bloom continuous casting facilities and slab continuous casting facilities in which the cast strip cross-section is large, the weight of the cast strip 32 and dummy bar itself is also great, and the falling incidence seriously damages the facility and foundation following below the cast strip withdrawing apparatus 33. Thus, the retaining force is preferably set to be large to allow for a margin, and it is required that a predetermined retaining force is maintained even if a rupture in a hydraulic hose in the hydraulic circuit or an oil leak from the plumbing arises.
On the other hand, contrary to the cast strip 32, the dummy bar is formed by machining, and thus the surface thereof is flat and smooth, and the coefficient of friction is low. While hot, the coefficient of friction between the cast strip 32 and both of the rolls 37 and 38 is on the order of 0.2 to 0.3; whereas, the coefficient of friction between the dummy bar and both of the rolls 37 and 38 is merely on the order of 0.1 to 0.15. In addition, the lubricant supplied to the bearings supporting both of these rolls 37 and 38, and the bearings supporting the support rolls of the roll segment having several support rolls adheres to the surface of the rolls 37 and 38 and the surface of the dummy bar; therefore, this coefficient of friction tends to be further lowered. Therefore, in order to reliably retain the dummy bar and the cast strip 32 following this, it is necessary to either sufficiently raise the pressing force of the hydraulic cylinder 39, or increase the number of drive rolls.
If the pressing force of the hydraulic cylinder 39 is increased, the retaining force can be raised. However, the surface pressure of the contact surfaces between the dummy bar and both of the rolls 37 and 38 becomes larger with a higher margin in the retaining force. As a result, permanent strain will occur in the dummy bar surface and the surface of both rolls 37 and 38, which will harm the life span of these. Notably, any of the rolls 37 and 38 will result in breaking from the repeated bending stress occurring on the rolls 37 and 38.
In order to raise the pressing force of the hydraulic cylinder 39, the pressure of the hydraulic oil may be raised. However, a pressure of 210 MPa is the practical limit. In order to further raise the hydraulic cylinder pressing force, increasing the diameter of the hydraulic cylinder cannot be avoided. If this is done, the hydraulic cylinder 39 will increase in size and will not be able to be housed in the cast strip withdrawing apparatus 33. In order to avoid this shortcoming, the number of both the rolls 37 and 38 may be increased; however, since the drive-train as well as the hydraulic system will increase, the cast strip withdrawing apparatus 33 itself will become larger scale.
The upstream-most side cast strip withdrawing apparatus and downstream side cast strip withdrawing apparatus are configured by two pairs of fixed-side rolls 37 and pressure-side rolls 38, respectively; however, the trouble of not being able to maintain the pressing force of the hydraulic cylinder 39 may occur from a rupture of a hydraulic hose of the hydraulic cylinder 39 or leakage of the plumbing, for example. Since the two pairs of pressure-side rolls 38 and 38 are configured by a common hydraulic system, the retaining force of both of the two pairs of pressure-side rolls 38 and 38 will become zero due to the occurrence of a rupture or leakage at one point. And the dummy bar and cast strip 32 following this will fall.
In order to solve this, if an individual hydraulic system is configured for each pair of pressure-side rolls 38 and 38, the pair of pressure-side rolls 38 and 38 are not affected by other hydraulic systems. However, since the retaining force will be halved in this case as well, the dummy bar and cast strip 32 following this will fall. In addition, configuring individual hydraulic systems for each pair of the pressure-side rolls 38 and 38 is not practical from a cost standpoint. In order to solve this, a method routinely using either hydraulic cylinder 39 and 39 of the pressure-side rolls 38 and 38 with its pressing force set to twice that required can be considered; however, since the surface pressure of the contact surfaces between the dummy bar and both rolls 37 and 38 is normally large, permanent strain is produced in the dummy bar surface and the surface of both rolls 37 and 38, and thus the life span thereof is harmed. Since large bending stress repeatedly acts on the pressure-side rolls 38 and 38, there is also risk of fatigue breaking.
Next, a cast strip anti-drop device for a continuous casting facility according to conventional example 2 will be explained hereinafter while referring to the appended FIG. 11. This cast strip anti-drop apparatus includes an emergency valve unit 58 that, under abnormal situation, ensures, by way of an emergency hydraulic circuit 56, high pressure for the oil pressure of a normal hydraulic valve unit 57 of a hydraulic cylinder 54B of a pinch roll 53B and aims to prevent falling of a cast strip 51 (refer to Japanese Examined Patent Application Publication No. S59 (1984)-29350).
However, since the oil pressure of a plurality of hydraulic cylinders (54A to 54F) is secured by the one emergency valve unit 58, if one hydraulic cylinder system ruptures or leaks, the other hydraulic cylinder systems will also experience a pressure drop at the same time, and thus the anti-drop function will not operate. For example, if the system of the hydraulic cylinder 54B ruptures, the pressure in the hydraulic lines connected to the hydraulic cylinders 54A, 54C, . . . 54F will also steeply drop due to the sudden leakage. Although it is effective for a leak on the order that is covered by the volume of an accumulator 59, or the securing of a pressure drop due to an internal leak of the hydraulic system during an electrical power failure, it is ineffective in the rupture of a hydraulic hose or a large leakage incidence.
In addition, according to a cast strip and dummy bar anti-drop hydraulic circuit for a continuous casting machine according to conventional example 3, when the hydraulic system of a pressing cylinder 66 that presses a pinch roll 63 to the dummy bar 64 ruptures, as shown in FIG. 12, the influence of a pressure drop on another hydraulic actuator group 67 can be prevented by isolating an electromagnetic isolation valve 68, (refer to Japanese Examined Patent Application Publication No. S61 (1986)-28425).
However, when a hydraulic system in the hydraulic actuator group 67 ruptures, the influence of the pressure drop on hydraulic actuators on an upstream side thereof cannot be prevented. In other words, this conventional example 3 is effective in preventing a steep pressure drop due to a rupture in the hydraulic system of the hydraulic cylinder 66, but is ineffective in a rupture in a hydraulic system other than the system of the hydraulic cylinder 66. In addition, according to FIG. 12, the two pressing cylinders 66 and 66 branch and are connected to one hydraulic line leading to and from the hydraulic actuator group 67; therefore, if the hydraulic line of one pressing cylinder 66 is ruptured, the hydraulic line of the other pressing cylinder 66 will also experience a sudden pressure drop at the same time, and the dummy bar 64 will suddenly fall.